Sacrificial cathodic protection is a technique that is used to limit the corrosion of steel in concrete. It involves connecting a base metal that is less noble than steel, such as a metal or alloy of zinc, aluminium or magnesium, to the steel. The base metal is consumed by anodic dissolution and in the process a current flows to the steel which becomes the protected cathode of the base metal—steel couple. U.S. Pat. No. 6,193,857 shows one arrangement that may be used to achieve this.
U.S. Pat. No. 6,685,822 discloses the use of sacrificial anodes in new concrete construction to protect steel in concrete. The sacrificial anode delivers current to the steel before the concrete has hardened to increase the tolerance of the reinforced concrete to aggressive chloride ion contamination.
Sacrificial anode systems exist as surface applied systems or embedded discrete systems. Surface applied anodes are large surface area anodes that deliver relatively low current densities of the order of 10 mA/m2 when expressed per unit of anode area. Discrete anodes are individually distinct compact anodes that deliver relatively high current densities of the order of 50 to 250 mA/m2 off the anode surface. They are placed in holes in the concrete or are attached to exposed steel in new construction or exposed steel at locations where patch repairs to the concrete are undertaken.
Discrete anodes are usually combined with an activating agent. The activating agent in contact with the base metal in a sacrificial anode assembly may prevent the anode from drying out or prevent the formation of insoluble products that restrict the dissolution of the base metal. U.S. Pat. No. 6,022,469 describes the use of KOH and LiOH to prevent the formation of insoluble zinc products that may otherwise result in zinc passivation. U.S. Pat. No. 6,165,346 describes the use of LiNO3 as a deliquescent material to prevent the anode from drying out. U.S. Pat. No. 6,217,742 describes the use of combinations of LiNO3 and LiBr to enhance the anode output.
In surface applied anode systems the source of protection current is distributed across the surface of the concrete. Current distribution is more complex with discrete sacrificial anodes. Cement & Concrete Composites vol. 24 (2002) pp. 159-167 investigates current distribution from a surface applied anode to embedded steel bars and notes that current distribution is affected by the boundary conditions, the concrete resistivity and the layout of the anode and the steel in the concrete.
The connection between the sacrificial anode and the steel reinforcement provides a path for electron conduction between the base metal and the steel. When the concrete is largely intact, the anode may be secured to the concrete surface or within the concrete cover and an electrical cable may be used to connect the anode to the steel. Such methods are also traditionally used to protect steel with other covering materials such as soil. The electrical cable may be connected to the steel using a clamp, clip or drilled and tapped hole. In addition, U.S. Pat. No. 6,572,760 describes a connection detail that involves obtaining contact between the anode and the reinforcing steel by impacting the anode against the steel in a suitably sized hole drilled through the concrete to the steel. U.S. Pat. No. 6,572,760 also describes a welded connection between the anode and the steel in a hole drilled through the concrete to the steel.
In some cases the concrete cover is not present and the steel is exposed. This occurs in new construction prior to casting the concrete and in existing construction when patch repairs are undertaken to corrosion damaged and spalled areas of a reinforced concrete structure prior to placing the cementitious patch repair material. In this case it is preferable to use the connection to secure the anode in place as well as make an electrical connection to the exposed steel. Tie wires have traditionally been used to secure and electrically connect the steel bars in reinforced concrete cathodic protection systems. The tie wire is typically a bare steel wire that is wrapped around the bars and tightened by twisting the ends of the wire together to secure and connect the bars together. WO 9429496 discloses an alkali activated sacrificial anode connected to a wire that is wrapped around the steel to form an electrical connection to the steel. U.S. Pat. No. 6,193,857 describes a method of connecting the anode to the steel which involves forming the anode around a section of a ductile metal conductor and wrapping the exposed ends of the ductile metal conductor around the steel and twisting the ends together to tighten the connection. Small loops are provided at the ends of the long ductile metal conductors that may be used by a twisting tool to tighten the connection.
Current practice is to tie the anodes directly on the exposed steel bars prior to casting the concrete. This practice is promoted by the existing inflexible connection detail. However, it is shown in Example 1 later in this document that tying the anodes directly on the steel results in poor distribution of the protection current. This invention discloses an advantageous method of connecting discrete sacrificial anodes to the steel in concrete in situations where the steel bars are exposed such as in new construction and at areas where patches of damaged concrete have been removed.